The present invention generally relates to a liquid crystal switching device and, in particular, relates to one such device having means, disposed substantially perpendicular to the path of polarized components of a split incident light beam, for controllably reorienting the polarization thereof.
The use of optical fibers, particularly as a telecommunication transmission medium, has numerous advantages over existing telecommunication media. For example, optical fibers can sustain a broader bandwidth signal and hence, can convey larger quantities of information over the same period of time than existing media. Further, light waves used in optical fibers have even shorter wavelengths than the conventional microwaves commonly used in telecommunication systems. Thus, a reduction in the physical size of components is readily achievable. This size reduction further can result in an overall cost reduction for materials, packaging and manufacturing. Still further, optical fibers exhibit little or no electromagnetic or radio frequency radiation thus resulting in a negligible environmental impact. Conversely, optical fibers are relatively insensitive to electromagnetic and radio frequency interference from the surrounding environment.
To be viable, every telecommunication system must include some means for controllably redirecting a signal, or at least a portion thereof, to or from a transmission medium or between one, or more, such media. In the case of an optical telecommunication system the redirecting means is an optical switch. Currently, optical switches are generally mechanical in nature.
However, mechanical switches require relatively high driving power and are subject to wear, abrasion and fatigue. As a result, mechanical switches are prone to failure after repeated use. In addition, since a rather small optical fiber is usually displaced from alignment with one port fiber into alignment with another port fiber, mechanical switches can easily become expensive. One particular reason for this expense is the very small tolerances required to ensure the proper alignment between the optical fibers moved and the optical fibers associated with the ports.
Recently, liquid crystal optical switching devices have been proposed as an alternative to mechanical switches. Typical of such liquid crystal optical switches are those described in U.S. Pat. No. 4,201,422 issued to McMahon et al. on May 6, 1980; U.S. Pat. No. 4,278,327 issued to McMahon et al. on July 14, 1981 and U.S. Pat. No. 4,385,799 issued to Soref on May 31, 1983. Therein a plurality of liquid crystal optical switch designs are described wherein optical fibers are attached to the side angled surfaces of a pair of opposing trapezoidal prisms. The trapezoidal prisms are arranged with the bases thereof parallel and with liquid crystal material disposed therebetween.
Although the optical switches described in the above referenced U.S. patents have some advantages over conventional mechanical optical switches, one advantage being no moving parts, these liquid crystal switches are both difficult and expensive to manufacture since all the surfaces of the trapezoidal prism must be ground, lapped and polished to be optically flat and fixed at a precise angular relation to each other. As a result, the liquid crystal optical switches described in the above referenced patents are currently impractical for optical communication systems.
More recently, liquid crystal optical switching devices have been developed that are less expensive and considerably more amenable to mass production techniques. Typical of these are the devices described and discussed in U.S. patent application Ser. Nos. 795,157 and 795,154 both filed on Nov. 5, 1985 and assigned to the assignee hereof. These applications are incorported herein by reference. The switching devices discussed therein generally include polarization changing cells that are provided with an alignment layer that tilts the molecules on the inner surfaces thereof. The tilted molecular alignment layer orients the surface molecules of the liquid crystal material. Such an alignment layer is used to ensure that the molecules of the surface layers of the liquid crystal material are disposed, i.e. tilted, so that a light beam incident thereon impinges normal to a homogeneously aligned liquid crystal material. Such a molecular alignment layer can, however, frequently be difficult to fabricate without impurities at the precise desired angle for effecting the reorientation of the polarization of a light beam traversing that cell. Even when such a layer is provided with negligible impurities, the possibility remains for impurities within the liquid crystal material, per se, to cause perturbations in the reorienting of the polarization of the light beam impinging thereon.
Consequently, a liquid crystal switching device that eliminates the need for a molecular alignment layer for molecular tilting is highly desireable in order to improve the viability of such switching devices.